Add docs for contributing to Pico's default theme

.gitattributes

+/.github        export-ignore
+/.gitattributes export-ignore
+/.gitignore     export-ignore

.github/CONTRIBUTING.md

+Contributing to Pico
+====================
+
+Pico aims to be a high quality Content Management System (CMS) but at the same time wants to give contributors freedom when submitting fixes or improvements.
+
+By contributing to Pico, you accept and agree to the *Developer Certificate of Origin* for your present and future contributions submitted to Pico. Please refer to the ["Developer Certificate of Origin" section](https://github.com/picocms/Pico/blob/master/CONTRIBUTING.md#developer-certificate-of-origin) in the `CONTRIBUTING.md` of our main repository.
+
+Aside from this, we want to *encourage*, but not obligate you, the contributor, to follow our contribution guidelines. Having said that: we really appreciate it when you apply the guidelines in part or wholly as that will save us time which, in turn, we can spend on bugfixes and new features. Please refer to the [`CONTRIBUTING.md` of our main repository](https://github.com/picocms/Pico/blob/master/CONTRIBUTING.md) for our actual contribution guidelines.

.github/PULL_REQUEST_TEMPLATE.md

+<!--
+
+Developer Certificate of Origin
+===============================
+
+By contributing to Pico, you accept and agree to the following terms and conditions (the *Developer Certificate of Origin*) for your present and future contributions submitted to Pico. Please refer to the *Developer Certificate of Origin* section in Pico's [`CONTRIBUTING.md`](https://github.com/picocms/Pico/blob/master/CONTRIBUTING.md#developer-certificate-of-origin) for details.
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+-->

README.md

@@ -49,3 +49,15 @@ social:
       icon: chat
 ---
 ```
+
+Getting Help
+------------
+
+Please refer to the ["Getting Help" section](https://github.com/picocms/Pico#getting-help) of our main repository.
+
+Contributing
+------------
+
+Please refer to the ["Contributing" section](https://github.com/picocms/Pico#contributing) of our main repository.
+
+By contributing to Pico, you accept and agree to the *Developer Certificate of Origin* for your present and future contributions submitted to Pico. Please refer to the ["Developer Certificate of Origin" section](https://github.com/picocms/Pico/blob/master/CONTRIBUTING.md#developer-certificate-of-origin) in the `CONTRIBUTING.md` of our main repository.